It seems to refer to the "rendering backend" used to paint user interfaces, as in actual/native elements. The PDF linked to by @DuncanMak compares it performan

It seems to refer to the "rendering backend" used to paint user interfaces, as in actual/native elements. The PDF linked to by @DuncanMak compares it performance-wise to things like Cairo (the canvas library used by GTK+), Qt Raster, and SKIA GL). It seems to perform very well in comparison.